1. Field of the Invention
The present invention relates to chemical processes in which a processing chemical is supplied in the form of repeated pulses of a gas phase or vapor phase reactant. More particularly, the invention relates to chemical processes for producing a thin film on a substrate by subjecting the substrate to repeated pulses of gas or vapor-phase reactants.
2. Description of the Related Art
There are several vapor deposition methods for growing thin films on the surface of substrates. These methods include vacuum evaporation deposition, Molecular Beam Epitaxy (MBE), different variants of Chemical Vapor Deposition (CVD) (including low-pressure and organometallic CVD and plasma-enhanced CVD), and Atomic Layer Epitaxy (ALE), which is more recently referred to as Atomic Layer Deposition (ALD).
ALE or ALD is a deposition method that is based on the sequential introduction of precursor species (e.g., a first precursor and a second precursor) to a substrate, which is located within a reaction chamber. The growth mechanism relies on the adsorption of one precursor on active sites of the substrate. Conditions are such that no more than a monolayer forms in one pulse so that the process is self-terminating or saturative. For example, the first precursor can include ligands that remain on the adsorbed species, which prevents further adsorption. Temperatures are maintained above precursor condensation temperatures and below thermal decomposition temperatures such that the precursor chemisorbs on the substrate(s) largely intact. This initial step of adsorption is typically followed by a first evacuation or purging stage wherein the excess first precursor and possible reaction byproducts are removed from the reaction chamber. The second precursor is then introduced into the reaction chamber. The second precursor typically adsorbs and reacts with the adsorbed species, thereby producing the desired thin film. This growth terminates once the entire amount of the adsorbed first precursor has been consumed. The excess of second precursor and possible reaction byproducts are then removed by a second evacuation or purge stage. The cycle can be repeated so as to grow the film to a desired thickness. Cycles can also be more complex. For example, the cycles can include three or more reactant pulses separated by purge and/or evacuation steps.
ALE and ALD methods are described, for example, in Finnish patent publications 52,359 and 57,975 and in U.S. Pat. Nos. 4,058,430 and 4,389,973, which are herein incorporated by reference. Apparatuses suited to implement these methods are disclosed in, for example, U.S. Pat. No. 5,855,680, Finnish Patent No. 100409, Material Science Report 4(7) (1989), p. 261, and Tyhjiötekniikka (Finnish publication for vacuum techniques), ISBN 951-794-422-5, pp. 253–261, which are incorporated herein by reference. ASM Microchemistry Oy, Espoo, Finland, supplies such equipment for the ALD process under the trade name ALCVD™.
According to conventional techniques, such as those disclosed in FI Patent publication 57,975, the purging stages involve a protective gas pulse, which forms a diffusion barrier between precursor pulses and also sweeps away the excess precursors and the gaseous reaction products from the substrate. Valves typically control the pulsing of the precursors and the purge gas. The purge gas is typically an inert gas, for example, nitrogen.
In some ALD reactors, some or all of the precursors may be initially stored in a container in a liquid or solid state. Such reactors are disclosed in co-pending U.S. patent application Ser. No. 09/854,707, filed May 14, 2001, and Ser. No. 09/835,931, filed Apr. 16, 2001, which are hereby incorporated herein by reference. Within the container, the precursor is heated to convert the solid or liquid precursor to a gaseous or vapor state. Typically, a carrier gas is used to transport the vaporized precursor to the reactor. The carrier gas is usually an inert gas (e.g., nitrogen), which can be the same gas that is used for the purging stages.
One problem associated with such ALD reactors and other chemical processes that use solid or liquid precursors is that it is difficult to determine how much solid or liquid precursor is left in the container. For example, low pressure is often required to volatilize the solid or liquid and the precursor may be highly flammable, explosive, corrosive and/or toxic. As such, the container is usually isolated from the surroundings except for the gas inlet and outlet conduits during use. Conventional measuring devices positioned in the container can be damaged and/or are impractical. As such, the chemical process is typically allowed to continue until the supply of precursor is exhausted. Operating in this manner is generally undesirable because it allows the concentration of the precursor in the reactor to drop below an ideal concentration range when the source is about to become depleted. One solution is to calculate the rate of precursor removal. Based upon the calculation, the container can be changed before the precursor is expected to be exhausted. However, a safety margin is typically including in the calculation. This can result in unused precursor remaining in the container, such that refilling is performed prematurely and the reactor downtime is increased (i.e., the duration of reactor use between refilling is reduced).
Another method for determining how much solid or liquid precursor is left in a container is disclosed in U.S. Pat. No. 6,038,919. This method involves closing an outlet of the container to define a measurement volume. A metered amount of gas is delivered to the measurement volume, while the pressure in the measurement volume is monitored. The pressure is used to calculate the amount of precursor remaining in the container. This method also has several disadvantages. For example, it requires that the outlet of the container be closed, which increases the downtime of the reactor.
It is also possible for the various valves and conduits between the precursor container and the reactor to become damaged or worn out. This can result in contamination and CVD-type reactions between the precursors, thereby compromising the ALD process. Therefore, a need also exists for an improved method and apparatus for determining when the valves, conduits and connections in an ALD reactor are worn out or damaged, preferably before worn out or damaged parts lower the throughput of the reactor.